In pulsed ultrasonography, short bursts of ultrasonic sound pressure waves are generated, and an image is formed from the reflected sound waves (echoes). The sound waves are typically produced by piezoelectric transducers. When a sound wave encounters a change in density, part of the sound wave is reflected back to the transducers. The time for an echo to return indicates the distance to the change in density, and the magnitude of the echo indicates the magnitude of the change in density. In conventional imaging, an image is formed from echoes having the same frequency as the transmitted frequency. In harmonic imaging, an image is formed from echoes having a frequency that is an integer multiple (harmonic) of the transmitted frequency. Typically, for harmonic imaging, only the second harmonic (the frequency that is twice the frequency of the fundamental frequency) has a sufficient intensity to provide useful imaging.
Harmonics may be generated as a result of a sound wave traveling through an elastic substance. As a sound wave travels through a substance, the peaks and troughs of the pressure wave cause the substance to alternately compress and expand. During compression, density increases, and the sound propagates slightly faster. During expansion, density decreases, and the sound propagates slightly slower. In addition, some substances are not linearly elastic. In particular, body tissues compress less than they expand. As a result of sound propagation speed and nonlinear elasticity, the sound waveform becomes progressively more distorted (asymmetrical) as it propagates, and the distortion produces harmonics of the fundamental frequency. In some situations, there are advantages to generating an image from harmonics instead of generating an image from the fundamental frequency. For one, a higher frequency provides improved spatial resolution to enable imaging of smaller objects. In addition, for body tissue, there are artifacts generated by reverberations of the body wall, and by weak echoes from the edge of the sound beam. Because harmonics are generated deeper in the tissue, they are not affected by the body wall. In addition, harmonic intensity is proportional to the square of the fundamental intensity, so weak fundamental echoes generate very weak harmonics that have less effect on the resulting images.
Harmonics of the transmitted frequency may also be generated by contrast agents that are injected into a region of interest. The contrast agents contain very small bubbles that vibrate at twice the frequency of the transmitted frequency. As a result, the strongest signal from the body at twice the frequency of the transmitted frequency comes from the region of tissue where the bubbles are located. In the resulting images, the region of tissue where the bubbles are located has a high contrast relative to other tissue.
Multiple techniques are used to eliminate the fundamental frequency in the echoes for harmonic imaging. One technique is to use a band-pass or high-pass filter on the signals from the echoes. Another technique is to add the echoes from two opposite polarity waveforms.
FIG. 1 illustrates an example ideal signal 100 for driving a piezoelectric transducer for harmonic imaging. There is a first waveform, called an upright waveform, indicated by time period 102. At some time later, there is an inverted waveform, indicated by time period 104. The upright waveform has a positive pulse, indicated by time period 106, followed by a negative pulse indicated by time period 108. The inverted waveform has a negative pulse, indicated by time period 110, followed by a positive pulse, indicated by time period 112. Echoes from each transmitted upright waveform and each transmitted inverted waveform are generated when the waveforms encounter a change in density, or when the transmitted waveforms excite a contrast agent. An echo from the upright waveform may be digitized and stored in memory. Likewise, an echo from the inverted waveform may be digitized and stored in memory. The two echo waveforms may be added together, and any differences are assumed to be from distortions generated within the substance being imaged, or from harmonics generated by contrast agents.
For the technique in which echoes from two successive transmitted waveforms are added, it is important that any differences resulting from addition of echoes are generated by asymmetrical propagation within the substance being imaged, or are generated by contrast agents, and not from any differences in the original transmitted waveforms. Ideally, the upright waveform and the inverted waveform are perfectly mirror symmetrical. There is an ongoing need to improve the symmetry of transmitted waveforms for ultrasound harmonic imaging.